I ran across the passage below in the book "Battle Cry of Freedom". I hadn't read about, or heard of this specific use of the Spencer before.
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McPherson (p. 752) cites only to Hoehling, ed.,
Last Train from Atlanta, pp. 58-59 (1958)(quoted in the first post), which I do not have. However, the ultimate source is pretty clearly Joseph G. Vale's
Minty and the Cavalry (1886) (pages 322-323 in my 2017 Forgotten Books reprint). One significant problem, though, is that Vale was not in the assault, though it is possible he observed it from the high ground on the western shore before his brigade followed.
The assault crossing, about a mile from Roswell, was made soon after dawn on July 9th by four companies of skirmishers, two each from the 17th and the 72d Indiana M.I., along a 300 yard front, supported by three companies of sharpshooters (two from the 17th and one from the 72d) deployed on the bluffs on the western side, and followed at distance by the rest of Wilder's Brigade (3d Bde, 2d Cav. Div.) and ultimately Minty's brigade (1st Bde). According to the report of Col. A.O. Miller, commanding Wilder's Brigade:
"The main column having been moved as close to the river as possible, and everything being in readiness, I ordered the skirmishers forward, and every man moved promptly into the water, when the enemy opened with a heavy fire, which was vigorously replied to by our sharpshooters from this side, and which attracted their attention from the men in the water. The river was running very swift, with a rough bottom, and in some places, up to the arms in depth, but the skirmishers moved steadily forward, keeping a good line, and before they reached the opposite shore the enemy fled in confusion, with the exception of a few who were captured before they could escape. The main column was at once moved forward to support our gallant skirmishers, but before it had crossed the advance had gained the crest of the ridge, 300 yards from the river. The object of the movement being accomplished, I formed my brigade in line of battle upon the ridge, Colonel Minty (the First Brigade) crossing immediately and forming upon my left." (O.R. vol. 38, part II, p. 851).
Minty's laconic report is consistent ("July 9, dismounted and waded the Chattahoochee in rear of the Third Brigade; threw up breast works and held the ground until dark, when General Newton's division, of the Fourth Corps, relieved us.") (id. p. 813). Vale was a captain in the 7th Penna. Cav. of Minty's brigade and also served as Inspector, First Brigade. Brig. Gen. Kenner Garrard, commanding the Second Cavalry Division, was equally brief ("On the 9th, in the presence of the enemy, the river was crossed, a foothold gained on the south bank, and the important ford at Roswell secured for our army.") (id., p. 804)
Benjamin F. Magee of the 72d Indiana, who was likely in the main column, provides this more florid description in his 1882
History of the 72d Indiana Volunteer Infantry of the Mounted Lightning Brigade, pp. 335-336, that is somewhat supportive of Vale:
"By this time our men are half way across the stream, and begin to open fire on the rebels in the brush along the shore. As our men rush up out of the water and immediately open fire with their Spencers, the river is all ablaze, the rebels think that pandemonium has broken loose,... and they break and run up the hill, our men shooting them in the back. Now there is a race who of our brave men shall reach the shore first. The water has gradually become shallower, and now is scarcely to their hips, and with such another running, shooting, cheering, and splashing, was never witnessed before. [names who "of those four companies" may have reached shore first] As our men reach the shore they rush for the top of the hill and capture three or four prisoners as they ascend, who are blanched and trembling with fright,
declaring that our men had just raised up out of the water and commenced to shoot at them." (emphasis added)